Expected Constraints on the Intergalactic Magnetic Field using Gamma-Ray Bursts with the Cherenkov Telescope Array Observatory

TL;DR

This paper addresses constraining the Intergalactic Magnetic Field (IGMF) using time-delayed very-high-energy gamma rays from Gamma-Ray Bursts (GRBs) observed by the Cherenkov Telescope Array Observatory (CTAO). It employs 3D electromagnetic cascade simulations coupled to CTAO instrument response and a joint spectral-temporal likelihood fit to extract the IGMF strength $B$ and spectral parameters, applied to GRBs 190114C and 221009A. The results indicate CTAO can probe $B$ down to about $10^{-16}$ G for GRB 190114C and up to about $10^{-15}$ G for GRB 221009A under conservative assumptions, with LST-1 already excluding $[3 \times 10^{-18}, 3 \times 10^{-17}]$ G for the latter. Accounting for realistic observing constraints, the study shows significant potential to tighten current lower bounds and test inflationary or chiral magnetogenesis scenarios; a non-detection would constrain early-Universe physics, while a detection would offer crucial insights into primordial magnetism.

Abstract

The InterGalactic Magnetic Field (IGMF), which could permeate the cosmic voids but was never detected so far, is considered a relic of the early Universe. Constraints on its strength $B$ can be derived from its influence on time-delayed very-high-energy photons from Gamma-Ray Bursts (GRBs) in the electromagnetic cascades along their path to the Earth. The present lower limit achieved on its intensity is $10^{-18}\;\mathrm{G}$. In this work, we simulate data from the Cherenkov Telescope Array Observatory (CTAO), accounting for realistic observational constraints, and we apply a joint spectral and temporal fit to characterise the IGMF. GRBs 190114C and 221009A are used as test cases to assess the sensitivity of CTAO. They demonstrate that a broad range of IGMF strengths can be probed with a lower bound as high as $10^{-15}\;\mathrm{G}$. Notably, we show that observations by the CTAO first large telescope, LST-1, already allow us to exclude field strengths up to $3\times 10^{-17}\;\mathrm{G}$.

Figures (3)

• Figure 1: Schematic representation of the cascade process in the one-generation approximation: primary photons generate electron-positron pairs at a distance $\lambda_{\gamma\gamma}$ from the source. These pairs are deflected by the IGMF by an angle $\delta$, and subsequently emit secondary photons that are observed at an angle $\theta$ relative to the line of sight.
• Figure 2: Fit map at $\lambda_B=1\;\mathrm{Mpc}$. The x-axis represents the injected values of $B$, and the corresponding best-fit values are shown as black dots on the y-axis. The colour scale denotes the confidence level associated with each fit. If every fit was totally accurate, the black dots would all lie on the red diagonal $\log B_\mathrm{fit} = \log B_\mathrm{dataset}$.
• Figure 3: (a) Light curve of GRB 221009A with the primary emission (black line) and the cascade for various $B$ (colours). The LST-1 upper limits are shown in purple. (b) Fit map at $\lambda_B=1\;\mathrm{Mpc}$.